The need for high performance semiconductor chips has continued to increase over the past several years. As demand for better performance and faster semiconductor chips has grown, so have efforts to reduce semiconductor chip size. Reducing the size of individual integrated circuit components can dramatically increase the speed and performance of a semiconductor chip. For example, smaller gate lengths in MOS transistors dramatically increase their switching speed.
The performance of semiconductor chips is limited by the electrical conductivity of the metal interconnects that electrically connect the various components that form the integrated circuits on the semiconductor chip. Therefore, in order to take full advantage of transistors that are capable of operating at faster speeds, the electrical interconnects must be highly conductive, yet low in resistance. Aluminum or an aluminum alloy was widely used as the preferred material for forming conductive interconnects by a process known as “metallization”. “Metallization” is a term used in the semiconductor industry to describe the process of wiring together the components of an integrated circuit, and typically occurs by the process of electrochemical deposition. Aluminum emerged as the preferred metal for metallization because it has a relatively low resistivity, good current-carrying density, and superior adhesion to silicon dioxide. In addition, aluminum is available in high purity and has a natural low contact resistance with silicon.
However, aluminum and aluminum alloys present some disadvantages when used to make interconnects. For example, although aluminum has a resistance that can be tolerated by most integrated circuits, it is difficult to deposit in a high aspect ratio. In addition, aluminum and aluminum alloys suffer from eutectic formations, thermally induced voiding, and electromigration when used in very large scale integration (VLSI) and ultra large scale integration (ULSI) semiconductor chips.
Recently, copper metal has begun to replace aluminum and aluminum-silicon alloys in VLSI and ULSI metallization processes. Copper has better conductivity and is more reliable than aluminum or aluminum alloys. The use of copper for making semiconductor chip interconnects is appealing because of the low cost, high throughput, excellent via-filling capabilities, and high quality of the deposited copper film. In contrast to aluminum, copper is capable of being deposited with high aspect ratios. Copper is also a much better conductor than aluminum, provides good step coverage, is more resistant to electromigration and can be electrochemically deposited at low temperatures.
Because copper will diffuse into silicon if applied directly to a silicon-containing substrate, such as a silicon wafer, it is necessary to first apply a barrier layer between the substrate from which the semiconductor chip is fabricated and the copper layer. Additionally, the material must have a relatively low resistivity so that electrochemical deposition of copper on the surface of the barrier layer will occur. Unless the resistivity of the barrier material is relatively low, it will not function as a suitable surface for subsequent electrochemical deposition of the conductive interconnect material. Tantalum, for example, has a resistivity that is two orders of magnitude higher than copper. Accordingly, tantalum is generally not suitable to serve as the cathode during the electrochemical deposition process.
To overcome this resistivity concern, a seed layer is typically formed on top of the barrier layer to provide the desired resistivity. For the electrochemical deposition of copper, the seed layer is often a layer of PVD copper. Unfortunately, formation of the seed layer overlying the barrier layer necessarily adds a discrete process step.
Accordingly, there continues to be a need in the industry for improved methods and devices for forming conductive layers, such as used in device interconnects, onto barrier layers.